Coking can be considered to be a severe thermal cracking process in which one of the end products comprise carbon, i.e. coke. The delayed coking process was initially developed to minimize refinery yields of residual fuel oil by severe cracking of feedstocks such as vacuum residuals and thermal tars to produce coke and lower molecular weight hydrocarbons. U.S. Pat. Nos. 4,049,538 and 4,547,284, the disclosures of which are incorporated herein by reference, show examples of delayed coking processes.
The delayed coking process generally involves heating the feedstock in the conduit or tubing of a tube heater to a temperature above the cracking temperature while feeding the feedstock at a high velocity through the conduit. The optimum operation involves the use of feed rate such as to minimize the actual formation of carbon in the heated conduit of the tube heater. The tube heaters are often referred to interchangeably as coker heaters or coker preheaters and the terms are similarly used interchangeably in this description.
In U.S. Pat. No. 4,049,538 a coker preheater is illustrated diagrammatically as item number 11. In U.S. Pat. No. 4,547,284 a coker heater is illustrated diagrammatically as item number 25. The heated feedstock at the coking temperature is passed from the heating zone to a coke drum wherein preferably the majority of the coke formation takes place. In the insulated coke drum, or surge drum, a sufficient residence time allows the coking to take place. Typically, the heated coking feedstock has been heated to a temperature sufficient to maintain the coking in the drum, i.e. temperature in the range of about 750 to about 975 degrees fahrenheit. As the process proceeds, coke accumulates in the coking drum and is later removed by techniques known in the art.
Although much effort has been devoted in the past to providing conditions which will allow for the delayed coking feedstock to be heated to the cracking temperature without the formation of undesirable carbon deposits in the conduit of the coker heater, carbon deposition in the conduits of the coker heater still continues to be a problem.
As coke deposits in the conduit of the tube heater, the flow of feedstock through the heater is restricted. The restriction of flow can lead to increased residence time which in turn can lead to the deposition of additional coke. The coke deposits in turn tend to insulate the tube so that more heat must be applied to achieve the same rate of heating of the feedstock. In addition the coke deposits cause the tubes to become much hotter. All these factors obviously tend to encourage the formation of still more coke within the tube of the tube heater.
If the temperature of the tube gets high enough, a tube rupture can occur. The likelihood of tube rupture is also aggravated by the fact that the feed must be pumped at ever higher pressures as the flow is restricted by coke deposition in the tubes of the heater. The combination of exposing the tubes to higher temperatures and higher pressures greatly increases the probability of tube rupture and total shut down of the delayed coking process.
Because of the formation of coke deposits in the tubes of the heaters, operators in the past have had to periodically shut down the operation and remove the coke that had been formed within the tubes of the heater.
It is believed that in all the prior art coking heaters the tubes through which the coking feedstock was passed have been secured to an inner side wall of the heater. The present invention is based in part upon the discovery that this prior art arrangement actually is a contributing factor to the formation of coke within the tubes of the heater.
An object of the present invention is to provide an improved delayed coking process in which the tendency for coke lay down in the tubes of the coking heater is greatly reduced.
Another object of the present invention is to provide a more efficient heater for a delayed coking process. Still another object for the present invention is to provide a coking heater which can be operated for extended periods of time without having to be taken off-stream for coke removal.
Still another object of the present invention is to provide a coking heater which can provide the desired level of heating with less overall length of heater tubing.
Still yet another object of the present invention is to provide a coking heater which allows for reduced residence time of the coking feedstock in the heater.
Other aspects, objects, and advantages of the present invention will be apparent to one having skill in the art from the following disclosure taken in conjuction with the drawings.